Polymeric protective foams (e.g. protective foam layers) are widely used for impact force attenuation in sports- and other safety-equipment. In general, a protective foam layer is placed adjacent or against a part of a person's body to protect that body part (e.g. the head) during an impact with, for example, the ground or even another person's head.
Protective foams function by absorbing and/or dissipating the impact energy from the force of an impact. An energy absorbing foam deforms or crushes upon impact thereby consuming a portion of the impact energy so that portion does not reach the underlying body part. An energy dissipating foam spreads the impact force over a larger surface area than the actual area of impact so that the force per unit area is decreased for the underlying body part compared to that for the initial impact surface (e.g. the outer surface of the protective layer or a hard outer shell over the protective layer).
All rigid or semi-rigid protective foams are energy dissipating foams to some extent because, due to their rigidity, they do not instantaneously yield upon impact. Instantaneous yielding would result in the transmission of the entire impact force to the localized region of the underlying body part immediately beneath the protective layer at the point of impact. Instead, rigid and semi-rigid foam layers typically retain sufficient rigidity during impact to transmit at least a portion of the impact energy from the point source (impact site) to lateral or adjacent regions of the foam layer before the energy is transmitted to the underlying body part. The result is to spread the impact force over a larger area and thereby reduce the force per unit area experienced by the underlying body part as described above.
However, conventional rigid and semi-rigid foams do not exhibit satisfactory energy absorption for many impacts, particularly for impacts at lower speeds, e.g. below 5 mph. For example, expanded polystyrene (EPS) is a rigid polymeric foam often employed as the protective foam layer in bicycle helmets. EPS is a rigid, non-resilient foam that may absorb impact energy by deforming or crushing during high speed impacts. This may prevent serious brain injury or even death in high speed bicycle accidents.
But, EPS is totally ineffective at absorbing the impact energy of low speed impacts (e.g. impact speeds lower than 5 miles per hour) where greater than 80% of all bicycling head impacts occur. This is because EPS is so rigid that it yields or crushes only slightly, if at all, at these low speeds. As a result, a rigid EPS protective layer as known in the art actually transmits most or all of the impact energy from low speed impacts directly to the underlying body part, typically the head.
There is a need in the art for a protective foam layer that provides effective impact energy absorption as well as dissipation at lower impact speeds, while still providing effective protection for high speed impacts. Preferably, such a protective foam layer will provide effective impact energy attenuation (both absorption and dissipation) across a range of impact speeds, from slow (e.g. less than 5 or 10 mph) to fast (e.g. greater than 15, 20 or 25 mph).